Method for manufacturing isolating structures

Abstract

A method for forming isolating structures in a silicon carbide layer includes depositing a masking layer on first and second portions of the silicon carbide layer, and forming openings through the masking layer to expose the first portions of the silicon carbide layer. Ions are implanted into the first portions of the silicon carbide layer. The silicon carbide layer is heated to form an oxide layer thereon having first portions on the first portions of the silicon carbide layer, and having second portions on the second portions of the silicon carbide layer. The first portions of the oxide layer are etched to form isolating regions in the silicon carbide layer.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to semiconductor processing, and more particularly, to a method for manufacturing isolating structures in a substrate. The substrate is particularly, but not exclusively, silicon carbide, and reference will be made throughout this description to this field of application for convenience of illustration.

BACKGROUND OF THE INVENTION

[0002] Silicon carbide is an extremely resistant material to chemical etching, as is well known to those skilled in the art. Removing a portion of silicon carbide by ordinary etching techniques as used in standard silicon-integrated circuit manufacturing processes is difficult. A method for etching away such layers is based on the use of a fluorine solution utilizing an especially dense plasma.

[0003] While being advantageous on several counts, this method has certain drawbacks. In particular, a hard mask layer must be used to define the areas to be removed. Defining the hard mask layer involves some serious resolution problems, and complicates the silicon carbide etching process.

SUMMARY OF THE INVENTION

[0004] In view of the foregoing background, an object of the present invention is to provide a method for manufacturing isolating structures in silicon carbide layers. This method is compatible with standard microelectronic device manufacturing techniques and is effective to overcome the limitations of the prior methods.

[0005] The principle on which this invention is based is one of “damaging” the areas of the silicon carbide layer to be removed by modifying the lattice structure of the silicon carbide layer using an ion implantation process, thereby making for faster and more efficient removal of such layers.

[0006] Based on this principle, the technical problem is solved by a method as previously indicated and as defined in the characterizing part of claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The features and advantages of the inventive method will be apparent from the following description of an embodiment thereof, given by way of a non-limitative example with reference to the accompanying drawings. In the drawings:

[0008] FIGS. 1 to 6 illustrate a first embodiment of the method according to the present invention;

[0009] FIGS. 7 to 13 illustrate a second embodiment of the method according to the present invention;

[0010] FIGS. 14 to 20 illustrate a third embodiment of the method according to the present invention; and

[0011] FIG. 21 is a graph showing the rate of growth of a SiC oxide layer plotted against time according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] With reference to FIGS. 1 to 6, a first embodiment of a method for manufacturing insulating structures, according to the invention, will now be described. The process steps and the structures described below do not form a complete process flow for manufacturing integrated circuits. In fact, this invention can be practiced jointly with integrated circuit manufacturing techniques currently used in the art, and only those common process steps will be discussed herein as are necessary for understanding the invention. The drawing figures which show cross-sections through a semiconductor wafer are not drawn to scale. They are drawn to highlight major features of the invention.

[0013] A masking layer 2 of a photoresist or metal, e.g., silicon oxide, is formed over the surface of a substrate 1 of silicon carbide (SiC). Openings 2A are formed through the masking layer 2 using conventional photolithographic techniques, and successive etching steps are performed for exposing the substrate portions where the isolating regions (trenches) 3A are to be formed.

[0014] In accordance with the invention, an ion implanting step is carried out to implant the chip surface with heavy ions or a dopant throughout. Thus, in the implanted regions 3 of the substrate 1 uncovered by the masking layer 2, the ion implanting step will result in the substrate 1 becoming “damaged”, in the sense that its Si—C lattice structure undergoes alteration. Advantageously in the inventive method, the depth of the “damaged” regions 3 can be varied by changing parameters, such as energy and dosage of the ions being implanted.

[0015] After removing the masking layer 2 from the entire surface of the substrate 1, the whole substrate 1 is subjected to an oxidizing step. An oxide layer 4 is formed over the chip surface. As is known, during the heat treating process, a surface portion of the substrate 1 is also converted into the oxide layer 4.

[0016] In particular, the oxide layer 4 has a first portion 5 with a first thickness in the regions 3, while on the rest of the substrate 1 the oxide layer has a second portion 6 with a second thickness smaller than the first thickness. The difference is due to variations in the lattice structure of the implanted regions 3, generated during the ion implanting step. This causes the rate of oxidation of the silicon carbide layer to increase, and consequently, the oxide layer 4 to grow thicker.

[0017] The process for thermally oxidizing the silicon carbide is extremely slow, as FIG. 21 illustrates. This figure shows a plot of the thickness of the oxide grown over SiC against time at different oxidation temperatures. Advantageously in this invention, the rate of oxidation increases substantially as ions are implanted in the SiC substrate 1.

[0018] For example, when silicon ions are implanted at 1 MeV and at a concentration of 5*1015 atoms/cm2, with the oxidation process being conducted at 1150° C. for two hours in an O2 atmosphere, the thickness of the oxide layer in the regions 3 is 2.5 &mgr;m, against an oxide layer that is 500 nm thick in the undamaged portion.

[0019] Under these conditions, the depth of the oxide layer in the substrate would be 1 &mgr;m when a conventional 1% fluorine technique is used. A step of removing the oxide layer from both the surface of the substrate 1 and the regions 3 results in isolating regions (trenches) 3A being defined depthwise in the substrate 1.

[0020] Advantageously in this invention, the isolating regions and their trenches are formed by etching through the layer of silicon oxide rather than the silicon carbide. Thus, standard removal operations, such as those provided in conventional integrated circuit manufacturing processes, can be carried out to form the trench regions 3A.

[0021] The method of this invention is especially advantageous where isolating trenches are to be formed for an epitaxially grown diode. This will now be described as a second application of the inventive method with reference to FIGS. 7 to 13. In the structure shown in these figures, identical elements with those of the structure previously illustrated are denoted by the same reference numerals.

[0022] A first epitaxial layer 7 is doped with impurities of the N-type at a relatively low dopant concentration, and a second epitaxial layer 8 is doped with P-type impurities at a relatively high dopant concentration. These layers are formed over the surface of a silicon carbide substrate 1. Advantageously, the second layer 8 is thinner than the first layer 7.

[0023] A masking layer 2 of a photoresist or metal or silicon oxide is formed over the second epitaxial layer 8. Using conventional photolithographic and etching techniques, the masking layer 2 is partially etched away to expose substrate portions where the isolating regions 3 will be formed.

[0024] Advantageously, the whole chip surface is ion implanted. In particular, heavy ions are implanted in the substrate portion where isolation is to be provided. As a consequence of the implantation, the structure of the layer 8 becomes “damaged” in the regions 3.

[0025] After removing the masking layer 2 from the entire surface of the substrate 1, the whole substrate 1 is subjected to an oxidizing step. Thus, an oxide layer 4 is formed, which has a first portion 5 with a first thickness in the regions 3, while in the epitaxial layer 8, it has a second portion 6 with a second thickness that is smaller than the first thickness.

[0026] Under these conditions, the depth of the first portion of the oxide layer 4 will be greater than the depth of the second epitaxial layer 8. Consequently, the second layer 8 is surface isolated from the edge of the structure and forms the anode of the diode. Advantageously, the oxide layer 4 can be removed completely and a suitable insulation material may be substituted.

[0027] An opening 4A is then formed through the oxide layer 4 such that at least a portion of the layer 8 is exposed. The diode manufacturing process is then completed by forming conventional metal layers 10. The method of this invention can also be applied to forming edge structures. For simplicity, a method of making an isolating structure for a diode as formed above will now be discussed with reference to FIGS. 7 to 13.

[0028] In particular, the process for manufacturing such an edge structure will be described starting from the endpoint of the step shown in FIG. 11, corresponding to FIG. 14 that only shows the peripheral portion of layer 8, as shown in FIGS. 14 to 20.

[0029] With reference to FIG. 15, the oxide layer 4 is removed completely to form a trench 4A. Subsequently, a mask 9, known as a ring mask, is formed on the chip edge. This is followed by an implanting step that affects the whole chip. Advantageously, this implantation is provided at an inclination angle effective to produce an implanted region 11 that extends across the bottom and the sidewalls of the trench 4A. After removal of the ring mask 9, the method comprises carrying out an oxidation process all over the chip to form an oxide layer 12 for diode edge isolation.

[0030] To summarize, when the method of this invention is applied, isolating structures or trenches can be formed in the layers of silicon carbide by standard integrated circuit manufacturing techniques. In particular, by providing for portions of a silicon carbide layer to be damaged, as by implantation with heavy ions or dopant ions, the inventive method effectively improves the rate of oxidation of the silicon carbide layer. As a result, the process of removing the oxidized carbide layer is compatible with standard removal techniques for integrated circuits, making for faster and more efficient removal operations.

Claims

1. A method for forming isolating structures in a silicon carbide layer (1,8), which method comprises the following steps:

depositing a masking layer (2) onto the silicon carbide layer (1,8); and

forming openings (2a) through said masking layer (2) to expose portions of the silicon carbide layer (1,8); characterized in that it further comprises the steps of:

ion implanting the whole chip surface;

heat treating the entire surface of the silicon carbide layer (1,8) to form an oxide layer (4) having a first portion (5) with a first thickness in said at least one region (3), and having a second portion (6) with a second thickness at said silicon carbide layer (1,8).

2. A method for forming isolating structures according to claim 1, characterized in that it further comprises the step of:

etching through said oxide layer (4) to form isolating regions (3A) in said first portion (5) of said oxide layer (4).

3. A method for forming isolating structures according to claim 1, characterized in that said masking layer (2) is removed completely before said heat treatment step.

4. A method for forming isolating structures according to any of the preceding claims, characterized in that said implanting step comprises implanting heavy ions.

5. A method for forming isolating structures according to any of the preceding claims, characterized in that said implanting step comprises implanting dopant ions.

6. An isolating structure as obtained by the method of any of the previous claims.